1. Field of the Invention
This invention relates to catalyst system for the polymerization of olefins, particularly, to a catalyst system comprising a conventional supported Ziegler-Natta catalyst and a lithium cyclopentadienide or lithium indene compound used to polymerize a-olefins, such as propylene.
2. Description of the Prior Art
Polyolefin manufacturing processes typically involve the polymerization of olefin monomer with an organometallic catalyst of the Ziegler-Natta type. Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst component; a co-catalyst, usually an organoaluminum compound; and an external electron donor compound or selectivity control agent, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl, typically an organoaluminum compound, as a co-catalyst. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum co-catalyst.
Lithium cyclopentadienide and lithium indene compounds are known in the synthesis of metallocene compounds useful as catalysts. U.S. Pat. Nos. 5,168,111; 5,096,867; 5,057,475 and 5,055,438 disclosed using a cyclopentadienyl lithium compound in a synthesis of a bridged metallocene-alumoxane catalyst system for the production of polyolefins and ethylene-a-olefin copolymers. U.S. Pat. No. 5,120,867 disclosed use of lithium indene in the synthesis of a silicon-bridged metallocene.
Lithium compounds have been used in hydrogenation of conjugated diolefin polymers. U.S. Pat. No. 4,673,714 disclosed the use of hydrocarbon lithium compounds, specifically alkyl lithium, with bis (cyclopentadienyl) titanium compound to hydrogenate the unsaturated double bonds of conjugated diolefins and an improved process which eliminated the need for these alkyl lithium compounds.
Molecular weight can be increased in polymerization processes using conventional supported Ziegler-Natta catalysts in several known ways.
It would be advantageous to increase molecular weight in polymerization processes using conventional supported Ziegler-Natta catalyst by the addition of a compound which interacts or modifies the catalyst without changing process condition or other catalyst components.
Accordingly, this invention provides a catalyst system to enhance molecular weight of polyolefins.
And, this invention provides a catalyst system in which increasing a component increases the molecular weight.
Also, this invention provides a polymerization process using a catalyst system to enhance the molecular weight of polyolefins.
Furthermore, this invention provides a process for making a catalyst for use in the polymerization of olefins to enhance molecular weight.
These and other objects are accomplished by a catalyst for polymerization of olefins comprising:
a) a conventional supported Ziegler-Natta catalyst component containing titanium;
b) a lithium compound of the formula LiCp wherein Cp is a cyclopentadienyl or substituted cyclopentadienyl;
c) an aluminum trialkyl co-catalyst described by the formula AlRxe2x80x23 where Rxe2x80x2 is an alkyl of from 1-8 carbon atoms and Rxe2x80x2 may be the same or different; and
d) an external electron donor either simultaneously with or after step (b), said external electron donor having the general formula SiRm(ORxe2x80x2)4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; Rxe2x80x2 is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with Rxe2x80x2; when m is 0, 1 or 2, the Rxe2x80x2 groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. Preferably, the molar ratio of LiCp/transition metal is at least 0.2.